One thing that most of these drugs do not do, despite the ingrained usage of the term hallucinogen, is to cause hallucinations. Hallucinations, strictly speaking, are perceptions that have no basis in reality, but that appear entirely realistic. A typical "hallucination" induced by a psychedelic drug is more accurately described as a modification of regular perception, and the subject is usually quite aware of the illusory and personal nature of their perceptions. Some less common drugs, such as DMT and atropine, may cause hallucinations in the proper sense.

Psychedelics, dissociatives, and deliriants have a long history of use within medicinal and religious traditions around the world. They are used in shamanic forms of ritual healing and divination, in initiation rites, and in the religious rituals of syncretistic movements such as União do Vegetal, Santo Daime, and the Native American Church.
When used in religious practice, psychedelic drugs, as well as other substances like tobacco, are referred to as entheogens.

Starting in the mid-20th century, psychedelic drugs have been the object of extensive attention in the Western world. They have been explored as potential therapeutic agents in treating depression, post-traumatic stress disorder, alcoholism, cluster headaches, and other ailments. Early military research focused on their use as incapacitating agents. Intelligence agencies tested these drugs in the hope that they would provide an effective means of interrogation. These drugs could incapacitate enemy troops, as well as friendly troops; in interrogation, they might induce a subject to talk, but they didn't consistently induce the subject to speak accurately and coherently.

Yet the most popular, and at the same time most stigmatized, use of psychedelics in Western culture has been associated with the search for direct religious experience, enhanced creativity, personal development, and "mind expansion". The use of psychedelic drugs was a major element of the 1960s counterculture, where it got associated with various political movements and a general atmosphere of rebellion and strife between generations.

Dr. Albert Hofmann, whose invention of LSD led to wide-spread Western interest in psychedelics

Perhaps because the psychedelic experience is so different from the experiences that our languages are commonly used to describe, much debate exists not only about the nature and causes, but even about the very description of the effects of psychedelic drugs. One prominent tradition involves the "reducing valve" concept, first articulated in Aldous Huxley's book The Doors of Perception.[1] In this view, the drugs disable the brain's "filtering" ability to selectively prevent certain perceptions, emotions, memories and thoughts from ever reaching the conscious mind. This effect has been described as mind expanding, or consciousness expanding, for the drug "expands" the realm of experience available to conscious awareness.

Yet the most remarkable characteristic of psychedelic effects is how much they vary depending on the precise drug, dosage, set, and setting. "Trips" range between the short but intense effects of intravenous DMT to the protracted ibogaine experience, which can last for days. Appropriate dosage ranges from extremely low (LSD) to rather high (mescaline). Some drugs, like the auditory hallucinogen DIPT, act specifically to distort a single sense, and others have more diffuse effects on cognition generally. Some are more conducive to solitary experiences, while others are positively empathogenic. Indeed, the popular drug MDMA ("ecstasy") has virtually none of the classical psychedelic effects, and is often thought of as purely empathogenic and in a different category from the psychedelics altogether.

Many psychedelics (LSD, psilocybin, mescaline, cannabis and numerous others) are extremely non-toxic, making it difficult to overdose unless these compounds are combined with other drugs.

Dissociatives are drugs which reduce (or block) signals to the conscious mind from other parts of the brain, typically (but not necessarily, or limited to) the physical senses. Such a state of sensory deprivation can facilitate self exploration, hallucinations, and dreamlike states of mind which may resemble some psychedelic mindstates. Essentially similar states of mind can be reached via contrasting paths -- psychedelic or dissociative. That said, the entire experience, risks and benefits are markedly different.

Many dissociatives also have CNSdepressant effects, thereby carrying similar risks as opioids to slowing breathing or heart rate to levels resulting in death (when using very high doses). This does not appear to be true in other cases, toxic effects do not appear to exist in the case of salvia divinorum, and the principal risk of nitrous oxide seems to be due to oxygen deprivation. Injury from falling is also a danger, as nitrous oxide may cause sudden loss of consciousness, an effect of oxygen deprivation. Long term use of dissociative anaesthetics such as PCP and Ketamine (and possibly DXM) have been suspected to cause Olney's lesions, though these lesions have never been demonstrated in primates to date.

The deliriants (or anticholinergics) are a special class of dissociative which are antagonists for the acetylcholine receptors (unlike muscarine and nicotine which are agonists of these receptors). Deliriants are considered to be true hallucinogens as users will have conversations with people who aren't there, or become angry with a 'person' mimicking their actions, not realizing it is their own reflection in a mirror (which could be dangerous if they became aggressive towards a glass mirror). While the regular dissociatives can produce effects similar to lucid dreaming (where one is consciously aware they are dreaming), the deliriants have effects akin to sleepwalking (where one doesn't remember what transpired during the experience).

In addition to the danger of being far more "out of it" than with other drugs, and retaining a truly fragmented dissociation from regular consciousness without being immobilized, the anticholinergics are toxic, can cause death due to overdose, and also include a number of uncomfortable side effects. These side effects include dehydration and mydriasis.

Hallucinogenic drugs are among the oldest drugs used by humankind, as hallucinogens naturally occur in mushrooms, cacti, and various other plants. Various cultures have endorsed the use of hallucinogens in medicine, religion, and recreation to varying extents, and some have regulated or outright prohibited their use. Today, in most countries, the possession of many hallucinogens, even those that are common in nature, is considered a crime and punished by fines, imprisonment, or even death. In the United States, some deference is given to traditional religious use by members of ethnic minorities such as the Native American Church. Recently the União do Vegetal, whose composition is not primarily ethnicity-based, won a Supreme Court decision authorizing its use of ayahuasca.

Historically, hallucinogens have been most commonly used in religious or shamanicrituals. In this context they are often referred to as entheogens, and they are used to facilitate healing, divination, communication with spirits, and coming-of-age ceremonies. Evidence exists for the use of entheogens in prehistoric times, as well as in numerous ancient cultures, including the Ancient Egyptian, Mycenaean, Ancient Greek, Vedic, Maya, Inca the and Aztec cultures. The Upper Amazon is home to the strongest extant entheogenic tradition; the Urarina of Peruvian Amazonia, for instance, continue to practice an elaborate system of ayahuascashamanism, coupled with an animistic belief system.

The rise of the Abrahamic religions (Judaism, Christianity and Islam) caused a decline of entheogenic use of hallucinogens use in its wake, as the authority of scripture and the priesthood gradually reduced the role granted to direct spiritual experience, especially by the laity. Examples of this development include the destruction of the Eleusinian Mysteries, which are now widely assumed to have involved entheogenic rituals, and the Great Witch Hunt of the Early Modern Age, in which practitioners of entheogenic rites in Western Europe were accused of associating with the Devil. The Spanish conquistadores associated local entheogenic traditions of South America with heresy and satanism, and uprooted many of them, but nevertheless, some cultures there and elsewhere have kept their traditions alive to this day.

Although natural hallucinogenic drugs have been known to mankind for millennia, it was not until the early 20th century that they received extensive attention from Westernscience. Earlier beginnings include scientific studies of nitrous oxide in the late 18th century, and initial studies of the constituents of the peyote cactus in the late 19th century. Starting in 1927 with Kurt Beringer's Der Meskalinrausch (The Mescaline Intoxication), more intensive effort began to be focused on studies of psychoactive plants. Around the same time, Louis Lewin published his extensive survey of psychoactive plants, Phantastica (1928). Important developments in the years that followed included the re-discovery of Mexicanmagic mushrooms (in 1936 by Robert J. Weitlaner) and ololiuhqui (in 1939 by Richard Evans Schultes). Arguably the most important pre-World War II development was by Albert Hofmann's 1938 invention of the semi-synthetic drug LSD, which was later discovered to produce hallucinogenic effects, in 1943.

After World War II there was an explosion of interest in hallucinogenic drugs in psychiatry, owing mainly to the discovery of LSD. Interest in the drugs tended to focus on either the potential for psychotherapeutic applications of the drugs (see psychedelic psychotherapy), or on the use of hallucinogens to produce a "controlled psychosis", in order to understand psychotic disorders such as schizophrenia. By 1951, more than 100 articles on LSD appeared in medical journals, and by 1961, the number increased to more than 1000 articles[2]. Hallucinogens were also researched in several countries for their potential as agents of chemical warfare. Most famously, several tragic incidents associated with the CIA's MK-ULTRAmind control research project have been the topic of media attention and lawsuits.

At the beginning of the 1950s, the existence of hallucinogenic drugs was virtually unknown among the general public of the West. However this soon changed as several influential figures were introduced to the hallucinogenic experience. Aldous Huxley's 1953 essay The Doors of Perception, describing his experiences with mescaline, and R. Gordon Wasson's 1957 Life magazine article (Seeking the Magic Mushroom) brought the topic into the public limelight. In the early 1960s, counterculture icons such as Jimi Hendrix, Jim Morrison, Jerry Garcia, Joe Mungo, MIke Rosato,Samantha Onorato, Timothy Leary, Allen Ginsberg and Ken Kesey advocated the drugs for their psychedelic effects, and a large subculture of psychedelic drug users was spawned. Many people argue that psychedelic drugs played a major role in catalyzing the vast social changes initiated in the 1960s.
As a result of the growing popularity of LSD, and, some contend, establishment disdain for the hippies with whom it was heavily associated, LSD was banned in the United States in 1967.

After the fading from public sight as one of the many elements of the 1960s counterculture, hallucinogen use took a less visible but nevertheless persistent role in Western society in the 1970s and 1980s. In the 1990s and 2000s something of a revival of interest in the drugs has occurred. There are probably several important contributing factors to the resurgence.

One is the rise of dance-based rave and trance music culture, in which participants frequently use drugs such as the entactogen MDMA, and to a lesser extent, other hallucinogenic drugs such as LSD, magic mushrooms and ketamine, as an aid to inducing ecstatic or trance states of consciousness. A second major contributing factor to the revival of interest in hallucinogenic drugs has been the advent of the Internet and World Wide Web. This has made information pertaining to drugs much more accessible to the general public, provided a platform for advocacy that was not previously available, and has enabled otherwise isolated interested parties to communicate and exchange information and experiences.

As of 2007, most well known hallucinogens (aside from dextromethorphan, diphenhydramine and dimenhydrinate) are illegal in most Western countries. One notable exception to the current criminalization trend is in parts of Western Europe, especially in the Netherlands, where hallucinogenic mushrooms are considered to be so-called "soft drugs", along with cannabis. While the possession of soft drugs is technically illegal, the Dutch government has decided that using law enforcement to combat their use is largely a waste of resources. As a result, public "coffeeshops" in the Netherlands openly sell cannabis for personal use, and "smart shops" sell drugs like psilocybin mushrooms and ayahuasca (See Drug policy of the Netherlands).

Since the latter part of the twentieth century, this attitude has spread throughout Europe; many European countries no longer actively pursue anti-drug policies, and rarely enforce extant legal penalties for personal-use quantities of hallucinogenic drugs. This is especially true with mild hallucinogens such as cannabis, which is rapidly gaining acceptance in western Europe as a harmless and socially acceptable intoxicant, much as alcohol is considered throughout the West. Despite being scheduled as a controlled substance in the mid 1980s, ecstasy's popularity has been growing since that time in western Europe and in the United States.

Many connect the idea of being "high" or going through a psychedelic state, as having brain damage or going crazy. This is due to the effect of the drug which, in some cases, can be overwhelming. Effects of these drugs can mimic psychological conditions such as psychosis, schizophrenia, and thought disorder. However, this is largely a misconception of the psychedelic state. After many studies investigating its possible use as a "psychotomimetic" and decades of personal/spiritual use it has become apparent that the psychedelic state is wholly different from a psychotic state and thus is ill-compared to schizophrenia and other mental disorders.

Several medical and scientific people, including Albert Hofmann, advocate the drugs should not be banned, but should be strongly regulated and warn they can be dangerous without proper psychological supervision. [2]

Taking a hallucinogenic drug without knowledge can result in psychological trauma, and has occurred many times because many psychedelic drugs such as LSD have low dose and can easily be added to food or drink, similar to "date rape drugs" or Mickey Finns, and those who deliberately do that can be charged with assault. (These occurrences have created some urban myths such as the blue star tattoo myth).

Most psychedelics are not known to have long-term physical toxicity. However, amphetamine-like psychedelics, such as MDMA, that release neurotransmitters may stimulate increased formation of free radicals possibly formed from neurotransmitters released from the synaptic vesicle. Free radicals are associated with cell damage in other contexts, and have been suggested to be involved in many types of mental conditions including Parkinson's disease, senility, schizophrenia, and Alzheimer's. Research on this question has not reached a firm conclusion. The same concerns do not apply to psychedelics that do not release neurotransmitters, such as LSD, nor to dissociatives and deliriants.

No clear connection has been made between psychedelic drugs and organic brain damage; however, high doses over time of some dissociatives and deliriants have been shown to cause Olney's lesions in other animals, and have been suspected to occur in humans. Additionally, Hallucinogen persisting perception disorder (HPPD) is a diagnosed condition where some effects of drugs persist after a long time--although medical technology has yet to determine what causes the condition.

The class of drugs described in this article has been described by a profusion of names, most of which are associated with a particular theory of their nature.

Louis Lewin started out in 1928 by using the word "phantastica" as the title of his ground-breaking monograph about plants that, in his words, "bring about evident cerebral excitation in the form of hallucinations, illusions and visions [...] followed by unconsciousness or other symptoms of altered cerebral functioning." But no sooner had the term been invented, or Lewin complained that the word "does not cover all that I should wish it to convey", and indeed with the proliferation of research following the discovery of LSD came numerous attempts to improve on it, such as "hallucinogen", "phanerothyme", "psychedelic", "psychotomimetic", "psycholytic", "schizophrenogenic", "cataleptogenic", "mysticomimetic", "psychodysleptic", and "entheogenic".

The word "psychotomimetic", meaning "mimicking psychosis", reflects the hypothesis of early researchers that the effects of psychedelic drugs are similar to naturally occurring symptoms of schizophrenia, which has since been discredited. It remained for a long time somewhat of a shibboleth to be used in the titles of papers as a signal that the researcher disapproved of the casual use of a drug, but has now been displaced in the medical literature by "hallucinogen". The latter term is not entirely accurate, since hallucinations strictly speaking must be entirely realistic but have no basis in reality, while psychedelic effects are often better described as distortions of the ordinary senses.

While the word "psychotomimetic" is now outmoded, the theory it implies is still clearly visible in the World Health Organization's definition of a hallucinogen as "a chemical agent that induces alterations in perception, thinking, and feeling which resemble those of the functional psychoses without producing the gross impairment of memory and orientation characteristic of the organic syndromes." [3]

The word "psychedelic" was coined by Humphrey Osmond and has the rather mysterious but at least somewhat value-neutral meaning of of "mind manifesting". Its use is often associated with the notion, preposterous in contemporary mainstream culture, that the psychedelic experience might be desirable or even beneficial. The word "entheogen", on the other hand, which is often used to describe the religious and ritual use of psychedelic drugs, is associated with the (yet more preposterous to mainstream culture) idea that it could be relevant to religion. The words "entactogen", "empathogen", "dissociative" and "deliriant", at last, have all been coined to refer to classes of drugs similar to the classical psychedelics that seemed deserving of a name of their own.

Many different names have been proposed over the years for this drug class. The famous German toxicologist Louis Lewin used the name phantastica earlier in this century, and as we shall see later, such a descriptor is not so farfetched. The most popular names, hallucinogen, psychotomimetic, and psychedelic ("mind manifesting"), have often been used interchangeably. Hallucinogen is now, however, the most common designation in the scientific literature, although it is an inaccurate descriptor of the actual effects of these drugs. In the lay press, the term psychedelic is still the most popular and has held sway for nearly four decades. Most recently, there has been a movement in nonscientific circles to recognize the ability of these substances to provoke mystical experiences and evoke feelings of spiritual significance. Thus, the term entheogen, derived from the Greek word entheos, which means "god within," was introduced by Ruck et al. and has seen increasing use. This term suggests that these substances reveal or allow a connection to the "divine within." Although it seems unlikely that this name will ever be accepted in formal scientific circles, its use has dramatically increased in the popular media and on internet sites. Indeed, in much of the counterculture that uses these substances, entheogen has replaced psychedelic as the name of choice and we may expect to see this trend continue.

Hallucinogens can be classified by their subjective effects, mechanisms of action, and chemical structure. These classifications often correlate to some extent. In this article, they are classified as psychedelics, dissociatives, and deliriants, preferably entirely to the exclusion of the inaccurate word hallucinogen, but the reader is well advised to consider that this particular classification is not universally accepted. The taxonomy used here attempts to blend these three approaches in order to provide as clear and accessible an overview as possible.

Almost all hallucinogens contain nitrogen and are therefore classified as alkaloids. THC and Salvinorin A are exceptions. Many hallucinogens have chemical structures similar to those of human neurotransmitters, such as serotonin, and temporarily modify the action of neurotransmitters and/or receptor sites.

A classical classification, mainly of historical interest, is that of Lewin (Phantastica, 1928):

Class I Phantastica roughly correspond to the psychedelics, which is a more modern term usually used as synonym to "hallucinogen" by people with positive attitudes towards them. Here the term is used a bit differently to discriminate one particular class of hallucinogens which it seems to describe best. They typically have no sedative effects (sometimes the opposite) and there is usually a clearcut memory to their effects. These drugs have also been referred to as the "classical" hallucinogens.

Class II Phantastica correspond to the other classes in our scheme. They tend to sedate in addition to their hallucinogenic properties and there often is an impaired memory trace after the effects wear off.

Erowid is a web site dedicated entirely to providing information about psychoactive drugs, with an impressive collection of trip reports, materials collected from the web and usenet, and a bibliography of scientific literature

The Shroomery has detailed information about magic mushrooms including identification, cultivation and spores, psychedelic images, trip reports and an active community.

Third-Plateau.org is a comprehensive site detailing uses and risks associated with DXM